[Bug c++/80456] calling constexpr member function from volatile-qualified member function: error: ‘this’ is not a constant expression

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80456

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
With the volatile, cxx_eval_constant_expression sees
*(volatile struct A *) this;, A::test();
and it's not able to evaluate 'this' in it.  Without the volatile it only sees
A::test()
which it can evaluate.

The COMPOUND_EXPR comes from build_new_method_call_1:

 8827               /* In an expression of the form `a->f()' where `f' turns
 8828                  out to be a static member function, `a' is
 8829                  none-the-less evaluated.  */
 8830               if (TREE_CODE (TREE_TYPE (fn)) != METHOD_TYPE
 8831                   && !is_dummy_object (instance)
 8832                   && TREE_SIDE_EFFECTS (instance))
 8833                 call = build2 (COMPOUND_EXPR, TREE_TYPE (call),
 8834                                instance, call);